This invention relates to a color cathode ray tube apparatus in which a deflection yoke is used to cause the electron beams to raster scan across the screen and in which a convergence assembly on the neck of the tube adjusts the convergence of the beams.
With a delta-gun CRT color display, a large green fringe can become apparent on vertical lines on the extreme left-hand edge of the screen when displaying data. This problem is apparently due to magnetic pickup from the deflection yoke.
During line flyback of the raster scan, high frequency magnetic fields from the deflection yoke interact with the coils of the convergence assembly on the neck of the tube. The magnetic fields feed through the internal convergence structure in the tube and through the ferrite/coil structure of the convergence assembly.
The pulse induced in the assembly is spread because of the frequency response characteristics of the tube and convergence assembly structure, and is of a polarity to oppose the waveform being generated to achieve convergence of the beams. The spreading of the pulse causes the convergence waveform to be in error over the first few microseconds of the video signal. Although the error is small, it can be significant in high-precision color displays.